Renu Tripathi

Pattern discrimination using a bank of wavelet filters in a joint transform correlator

We introduce and demonstrate the use of wavelet-filter-based processing in a joint transform correlator for achieving high-quality target discrimination. The inherent frequency localization characteristic of these filters offers high discrimination in correlation for lookalike targets. Re- sults obtained from simulation and experimental studies validate this fact. Wavelet filters are generated and applied to the joint power spec- trum through fast digital processing and are displayed onto a spatial light modulator. The correlation results are analyzed for target discrimination. The hybrid experimental setup combines the speed of optical processing with the flexibility of digital processing to realize such a technique in real time.

Pattern discrimination using wavelet filters in a photorefractive joint-transform correlator

Abstract We propose and demonstrate the use of wavelet filters for pattern discrimination in a photorefractive (PR) joint-transform correlator. A PR BaTiO3 crystal has been used to record the joint power spectrum in real-time. Wavelet filters are generated in the form of a filter bank and are projected serially onto the crystal, using a liquid crystal spatial light modulator (SLM). Choice of a suitable filter offers a high degree of pattern discrimination.

In-plane displacement measurement using a photorefractive speckle correlator

Abstract The present work describes the use of a photorefractive correlator in the measurement of in-plane displacement. A correlation technique has been proposed and demonstrated for precise displacement measurement by recording objective speckles. Speckle interference patterns are generated in situ using a converging beam illumination and two-beam coupling recording geometry in a photorefractive barium titanate crystal. Intermediate digital interfacing through an electrically addressed spatial light modulator enables one to introduce digital processing techniques (viz., thresholding) for improving the fringe contrast resulting in a better quality of correlation output required for measurement accuracy and reliability. This also increases the range of displacement measurement. In-plane object displacement being proportional to the separation between the correlation peaks, a measure of it gives the magnitude of object displacement. Both simulation and experimental studies have been performed on double as well as multiple exposure speckle interference patterns. The effect of threshold binarization on speckle correlation has also been studied. The method promises high degree of accuracy and increased range for displacement measurement in real-time.

Object tilt measurement using a photorefractive speckle correlator: theoretical and experimental analysis

A speckle correlation technique is proposed for the measurement of small object tilt. Theoretical analysis is formulated for a general case of object tilt. Simulation and experimental studies are performed to verify the outcome of theoretical predictions and accuracy in measurement. Sensitivity of tilt measurement is studied and quantified from observations made from object position in various longitudinal planes.

Three-dimensional displacement measurement using chirp modulation in a photorefractive correlator

A speckle correlation technique based on chirp modulation of the speckle interference pattern is proposed and demonstrated for the measurement of 3-D rigid body motion. The chirp modulation is a con- sequence of the out-of-plane displacement component associated with object motion. The Fresnel zone plate action introduced by chirp modu- lation causes the correlation peaks to focus at different planes along the longitudinal direction. The separation between the correlation peaks along the transverse direction gives a measure of in-plane displacement of object motion. Both simulation and experimental studies are per- formed to establish the proposed technique.

Nonlinear processing and fractional-order filtering in a joint fractional Fourier-transform correlator: performance evaluation in multiobject recognition

We investigated the correlation performance of a joint fractional Fourier-transform correlator (JFRTC) using computer simulation results. We present a mathematical analysis suggesting use of processing techniques based on a nonlinear transformation and fractional-order fractional-power fringe-adjusted filter to attain improved performance in terms of discrimination sensitivity and input space-bandwidth utilization. Optimal noise performance for the JFRTC is predicted in the presence of additive white Gaussian noise. An all-optical implementation scheme based on incoherent erasure in a photorefractive crystal is proposed.

Photorefractive joint transform correlator using incoherent erasure in two- and four-wave mixing geometries in a BaTiO3 crystal

The outcomes of some experimental and simulation studies performed on an incoherent-erasure photorefractive joint transform correlator (PR JTC) in the two-wave- and four-wave-mixing geometries are demonstrated. Computer simulation results show the effectiveness with which nonlinear transfer characteristics can be achieved in the incoherent-erasure configuration of the JTC. These can be exploited to attain different filter operations, e.g., matched, phase-only, and inverse filters by varying beam ratios. Experimental limitations with PR barium titanate crystal entail the incoherent erasure (IE) JTC operation with all filter actions and encourage to this goal further investigations with fast PR materials.

Photorefractive joint-transform correlator using incoherent-erasure in two-wave-mixing geometry

We propose and investigate a photorefractive joint-transform correlator that uses an incoherent erasure beam in a two-wave-mixing arrangement. Computer simulation and experimental results are presented showing its performance in terms of discrimination ability and noise robustness. Simulation studies show that this JTC can exploit a nonlinear transfer characteristic inherent to the energy transfer process.

Noise immunity and pattern discrimination using wavelet feature-based generalized fringe-adjusted joint transform correlator

A wavelet feature-based generalized FAJTC has been proposed to improve the performance of fringe-adjusted type JTCs in the presence of noise in the input scene. Computer simulation results are produced for test images containing single as well as multiple targets and synthetic textured input scene noise. The results illustrate the effectiveness of wavelet processing in reducing the detrimental effect of noise and enhancing pattern discrimination ability of conventional generalized FAJTCs.

Polychromatic light-based photorefractive correlator for 3D motion measurement

We have proposed the use of polychromatic light based photorefractive correlator for 3D motion measurement. The double exposures of speckle patterns can be recorded in a BaTiO3 crystal, thus providing a set of interference fringes due to the polychromaticity of the light source and the 3D displacement. Due to the fringes corresponding to the two illuminating wavelengths used in our experiment, a set of correlation output is generated which helps in a better averaging of the results of measurement. This technique further contributes to the enhancement of accuracy and range of displacement measurements.